1. Field of the Invention
The present invention relates generally to systems and methods for controlling spacecraft or satellite, and in particular to a system and method for minimizing solar array sun tracking disturbances.
2. Description of the Related Art
Most three-axis stabilized spacecraft or satellites use two solar arrays to generate power for operation of electrical systems aboard the spacecraft. The solar array must be maintained in a position normal to the sun to absorb the optimum amount of radiation. For optimal performance, the planar surface of the solar array is typically maintained substantially normal to a vector from the spacecraft to the sun. This is accomplished by servo-controlled stepping mechanism, such as a stepping motor and an appropriate gear assembly, which rotates the solar array along its longitudinal axis to track the sun while the spacecraft orbits about the Earth. The rate that the solar array must be rotated is a function of the satellite orbital period, but is typically about 0.004 degrees per second.
The solar wing driver (SWD) typically includes a stepper motor coupled to the solar arrays via gear-driven transmission. Stepper motors are desirable because they are relatively simple to control, reliable, lightweight and well adapted to continuous use.
However, the use of a stepper motor with highly flexible solar arrays may potentially excite some structural modes of the solar array and generate significant oscillation disturbances in the spacecraft itself. These disturbances can degrade the spacecraft pointing, cause excessive activity of the spacecraft control actuators, and make autonomous spacecraft momentum dumping difficult. The induced oscillation is particularly critical in spacecraft where absolute platform stability is desirable. Vibrations can cause deterioration of any inertia-sensitive operations of a spacecraft.
This disturbance problem can be ameliorated by a number of techniques. One technique is to employ high bandwidth control loops to mitigate the impact of this disturbance to the spacecraft pointing. However, this technique has significant limitations. For many spacecraft, the structural modes that are excited by the SWD stepping is outside of the spacecraft control bandwidth. Consequently, these high-bandwidth control loops have only very limited effects on the disturbance. Further extension of the bandwidth of the control loops to include these structural modes will very often result in control loop stability problems. Furthermore, high bandwidth control also unnecessarily increases actuator operation, which can increase wear and result in excess energy consumption. Another technique for minimizing the solar array sun tracking disturbance is disclosed in U.S. Pat. No. 4,843,294, entitled “Solar Array Stepping to Minimize Array Excitation,” issued Jun. 27, 1989 to Bhat et al., which is hereby incorporated by reference herein. In this reference, mechanical oscillations of a mechanism containing a stepper motor, such as a solar array powered spacecraft, are reduced and minimized by the execution of step movements in pairs of steps (a two-step dead beat method). The period between steps is equal to one-half of the period of torsional oscillation of the mechanism. While this method can reduce structural disturbances, it is not effective when the mechanism has significant backlash and stiction. This is because the backlash and stiction can significantly interrupt the two-step pattern of this method and thus render it not very effective.
There is therefore a need for a robust system and method for minimizing disturbances in stepper-motor driven solar arrays and related components. The present invention satisfies that need.